I. Field of the Invention
This invention relates to directional drilling systems and, in particular, to a selectively adjustable bent motor housing string which provides aggressive kickoff angles from the primary well bore.
II. Description of the Prior Art
Directional drilling techniques have become increasingly important for maximizing drilling success while lowering costs. Directional drilling most basically involves kicking off at an angle from a vertical well bore until a desired angle and/or horizontal position is attained allowing drilling across petroleum deposits maximizing production through a single well. It is desirable to minimize the radius of the directional drilling by maximizing the build rate achieved by the bent motor string Factors which affect the build rate include the stiffness of the drill string, the length of the motor housing and the position of the contact points of the drill string in relation to the drill bit and the bend of the motor housing.
Past drilling systems have incorporated various means for directing the drilling from the main well bore. The most basic structures include one or more stabilizers, a drill bit and a bent motor housing (or, alternatively, a drilling motor with a separate bent housing). When straight downhole drilling is desired the entire string is rotated to work the bit. At the point of kickoff, rotation of the full string is stopped and the drilling motor only is operated The deviation angle build rate will depend upon the placement and size of the stabilizers which push off from the walls of the well bore to direct the drill bit. Eccentrically mounted stabilizers have been employed to try and increase the kickoff from the walls of the well. Although such assemblies provide good deviation angles, the angle build rate is limited by the size of the stabilizer pads which must fit in the well hole. Increased kickoff pads sizes require larger initial well bores to prevent premature deviation. Moreover, in the case of eccentric stabilizers it is desirable to position the kickoff pads of the lowermost stabilizers in opposite orientations to increase the kickoff angle. However, this increases the effective diameter of the drill string which could hang up in the hole.